The present disclosure relates to a fixing device configured to apply heat and pressure to an image formed on a recording paper sheet to fix the image thereon and an image forming apparatus.
Fixing devices are configured to press a heat roller and a pressure roller against each other to form a nip area between them and then nip a recording paper sheet in the nip area with the heat roller heated so that an image (unfixed toner image) on the recording paper sheet is fixed by the application of heat and pressure.
In order to reduce the heating time of the heat roller and save energy in such a fixing device, there is a tendency to reduce the thickness of the peripheral wall of the heat roller to reduce the heat capacity of the heat roller.
However, if the thickness of the peripheral wall of the heat roller is too small, the rigidity of the heat roller becomes low. Thus, the pressure of engagement of the pressure roller against the heat roller may deform the heat roller, in which case an appropriate nip area cannot be ensured. To cope with this, in the case of making the heat roller from aluminum, the peripheral wall of the heat roller is designed to have a thickness of 0.55 mm or more. In the case of making the heat roller from stainless steel, the peripheral wall of the heat roller is designed to have a thickness of 0.3 mm or more. In these manners, the rigidity of the heat roller are ensured.
Alternatively, a member configured to be pressed against the inner peripheral surface of the heat roller may be provided to prevent deformation of the heat roller and ensure a nip area between the heat roller and the pressure roller.
For example, a fixing device is known in which a fixed pad is pressed against the inner peripheral surface of a fixing roller (an equivalent of the heat roller), more specifically against a region thereof corresponding to a nip area, thus preventing deformation of the fixing roller.
Likewise, a fixing device is also known in which a second pressing member is pressed against a region of the inner peripheral surface of a fixing roller corresponding to a nip area to prevent deformation of the fixing roller. Also, there is a fixing device configured to prevent deformation of a heat roller by pressing a pressing member against near a region of the inner peripheral surface of the heat roller corresponding to a nip area.
Furthermore, there is known a fixing device configured to press a metal roller against a region of the inner peripheral surface of a fixing roller corresponding to a nip area. This metal roller is provided to homogenize the temperature distribution of the fixing roller but can also be expected to have the effect of preventing deformation of the fixing roller.
Likewise, there is also known a fixing device configured to press a highly thermally conductive roller against near a region of the inner peripheral surface of a heat roller corresponding to a nip area. This highly thermally conductive roller is also provided to homogenize the temperature distribution of the heat roller but can be expected to have the effect of preventing deformation of the heat roller.
In the case where, as in each of the above fixing devices, deformation of the heat roller is prevented by pressing a member against the inner peripheral surface of the heat roller, the thickness of the peripheral wall of the heat roller can be reduced to 100 μm to 200 μm. In this relation, a 0.55 mm or larger thickness of the aluminum-made heat roller or a 0.3 mm or larger thickness of the stainless steel-made heat roller, which are examples of enough thicknesses of the peripheral wall to ensure the rigidity of these heat rollers, can be achieved by cutting. However, a 100 μm to 200 μm thickness is difficult to achieve by cutting and has to be provided by rolling.